The near-infrared emission spectra of nanotube samples serve as compositional "fingerprints". Spectrofluorimetry can therefore be used to deduce detailed information about the compositions of bulk samples, which invariably contain mixtures of (n,m) species. Applied Nanofluorescence (ANF) has developed a specialized fluorimetric instrument designed specifically for single-wall carbon nanotubes (SWCNT) analysis. The unique fluorimetric analyzer combines a compact, efficient optical system and sophisticated software that acquires and interprets the data. It allows users to easily and rapidly find the identities and amounts of specific (n,m) semiconducting nanotubes in their bulk samples.

Near-IR fluorescence is not emitted by the one-third of SWCNT species that have metallic character, or from multi-walled carbon nanotubes (MWCNTs). However, these nanotubes can be detected and characterized by their optical absorption and Raman spectra. Absorption and Raman also offer valuable information about SWCNT samples. General-purpose spectrofluorometers are not optimized for nanotube analysis. Although versatile, they are bulky, slow, insensitive, and give raw data that need careful manual interpretation. In addition, they cannot measure absorption or Raman spectra. ANF has developed the NS1 and NS2 instruments specifically for nanotube analysis. These integrated systems efficiently measure and interpret multi-mode spectral data to provide the best available optical characterization.

Near-infrared band-gap fluorescence from individual semiconducting SWCNT was discovered at Rice University in 2001. Subsequent research at Rice deciphered the complex pattern of absorption and emission peaks seen in mixed samples. As a result, each distinct spectroscopic feature has now been assigned to a specific nanotube structure. These structural species differ in diameter and chiral angle, and are uniquely labeled by pairs of integers denoted (n,m).

Assessing SWCNT dispersion quality

Dispersed semiconducting SWCNTs can emit near-IR fluorescence if they are free of growth defects, have not undergone sidewall chemical reactions, and are individually suspended rather than aggregated into bundles with other nanotubes. The measured fluorescence intensity from a SWCNT dispersion therefore depends not only on optical excitation power and nanotube concentration, but also on the sample’s “quality.” To allow simple quantitative assessment of this quality, the NS3 NanoSpectralyzer automatically measures and reports the total (spectrally integrated) near-IR emission power from the sample for 3 different excitation wavelengths.

Determining (n,m) compositions of SWCNT samples

SWCNT samples almost always contain a number of (n,m) structural species, including semiconducting and metallic forms. The semiconducting SWCNT species present in a sample can be detected and identified by their fluorimetric signatures, which involve characteristic emission peaks in the near-IR and characteristic excitation peaks in the visible region. Two-dimensional excitation-emission scans were originally used to observe these features and qualitatively analyze for the semiconducting (n,m) species. The continuously wavelength-tunable excitation light needed for such scans is available only by using a monochromator to filter the output of a broadband lamp. Unfortunately, the resulting lamp-based excitation beam is low in power and impossible to focus tightly. The NS1 instead uses three higher power diode lasers with fixed wavelengths carefully chosen to excite a broad range of semiconducting SWCNTs.

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